1. Field of the Invention
The present invention generally relates to flip-chip packaging, and more particularly to a method of forming metallic bumps on the I/O pads and sealing up the active side of a semiconductor device.
2. The Prior Arts
Flip-chip packaging utilizes bumps to establish electrical contact between a chip's I/O pads and the substrate or lead frame of the package. Structurally, a bump actually contains the bump itself and a so-called under bump metallurgy (UBM) located between the bump and an I/O pad.
An UBM generally contains an adhesion layer, a barrier layer and a wetting layer, arranged in this order on the I/O pad. The bumps themselves, based on the material used, are classified as solder bumps, gold bumps, copper pillar bumps and bumps with mixed metals.
To form bumps on the UBMs, usually electroplating, printing, or stud bonding method is used. For electroplating, patterned resists are first formed on the UBMs and then metals are plated. For printing, solders are first printed on the UBMs and the solders are thermally cured into the bumps. For stud bonding, it is used for limited gold bumping only.
The semiconductor device with bumps will be soldered onto a substrate or lead frame. Then underfill is dispensed into space under the semiconductor device to seal up the active side of the semiconductor device and fix the semiconductor device to the substrate or lead frame firmly. Conventional flip-chip packaging process includes bumping, assembly, underfill dispensing and optional molding. Before underfill dispensing is completed, the bump only sits on UBM alone with a risky height/width ratio. Bump lift is the major failure in flip-chip packaging especially when bump count is high.